[Coco] Re: Help: Magnavox 1CM135 Monitor

[Richard Batt]

Boisy:

The most common problem on Magnavox monitors was cold solder
joints on the flyback transformer.  Marty Goodman used to
recommend resoldering all the connections on the flyback
when you have this type of problem.  I had to do this on my
Magnavox monitor, although I'm not sure if its exactly the
same monitor.  However,I understand this was a common problem
for most Magnavox monitors and small TV's.

The big problem with working on this transformer is it is
used to generate the high voltage for the CRT.  A CRT can
hold a charge for many days waiting for some unsuspecting
person to touch the wrong spot, and then ZAP.  It probably
won't kill you, but it could, and it's definitely unpleasant.
It's possible, since the monitor's not working, that there is
no charge left, but you can't depend on that.

If you are going to try repairing this, first discharge the
high voltage circuit.  I use a long screwdriver with a plastic
handle.  I hook a jumper wire with alligator clips on each end,
one end on the screwdriver near the handle, the other end on
the monitor chassis.  Someplace on the steel frame that every-
thing's mounted to.  Then stick the tip of the screwdriver
under the rubber shield that surrounds the wire that hooks to
the side of the CRT.  Pry the rubber up enough so you can see
underneath.  Touch the metal ring embedded in the glass of the
CRT and the end of the HV wire where it hooks into the CRT.
The CRT is what stores the charge, so if you get that discharged
you should be safe.  You might touch the terminals on the
flyback transformer where they come through the pc board just
to touch all the bases.

The flyback transformer is maybe 3 in. across and is mounted on
the pc board that runs vertically.  I forget which side of the
monitor its located.  However, the HV wire from the CRT runs
either right to it or next to it.  The transformer is heavy
enough that handling fractures the solder joints where its
soldered to the pc board.  If you casually look at the solder
joints they look OK.  But under a bright light and magnifying
glass you may be able to see the cracks.  When the solder first
cracks, everything keeps working because the broken parts
are still touching.  But eventually the solder oxidizes (just
like the tin plated contacts on disk controllers and other
packs) and loses contact.  And stops working.

Whether you can see cracks or not, I would recommend resoldering
all the pins of the flyback transformer where they come thru
the pc board.  Some of the pins are rather heavy, so you will
need a fairly heavy soldering iron, heavier than one would
normally use for normal pc board work.  Add a small amount of
new solder to each joint, but make sure all the solder flows
between the pin and board run.  If you get too much solder on
a pin, solder suck it all off and start over.

As this was a common problem, the odds are pretty good resoldering
the pins will fix it, but of course, it could be something else.

Hope this helps you out.

Dick

ps I went thru all my copies of Marty Goodman articles from
Rainbow and did a quick check of articles in the hardware
database from Delphi but couldn't find anything.  But I'm
positive that Marty wrote up something about working on
Magnavox monitors with this problem if you want to do some
hunting, yourself.

 
Original Message
------------------------------
From: Boisy G. Pitre <boisy at ...>
Subject: Help: Magnavox 1CM135 Monitor
Newsgroups: gmane.comp.hardware.tandy.coco
Date: Sun, 29 Feb 2004 10:21:03 +0000

I have a Magnavox 1CM135 RGB/Composite Monitor - this thing was working 
great, then all of a sudden stopped.  Now there is no picture.  Just a 
constant, high pitch squeal when you apply power.

Can any of you Monitor guys (Gene Heskett?) provide some insight as to 
what possibly may be wrong?  I don't have much expertise on diagnosing 
monitors, and before I pitch it to the curb, I want to make sure it 
isn't something simple or easy to fix.

Boisy



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